This invention generally relates to a method and apparatus for separating isotopes and, particularly, a two-photon photoexcitation process and apparatus for separating uranium isotopes.
A fissionable material is one that readily undergoes fission when struck by neutrons. The only naturally available fissionable material is uranium-235, an isotope of uranium constituting less than one percent of the naturally occurring element. Almost all the rest of the natural uranium element is the uranium-238 isotope. The complete or partial separation of the isotopes of naturally occurring uranium results in a product called "enriched uranium" which has a higher than normal concentration of uranium-235 and a waste product called "depleted uranium" which has a lower than normal concentration of uranium-235. The production of enriched uranium is a vitally important process for nuclear applications.
The most commonly known methods for uranium isotope separation are the gaseous diffusion process, which is currently being used in the United States, and the gaseous centrifugation process. Both of these processes require a tremendous amount of energy in order to separate relatively small amounts of uranium isotopes and thus are extremely expensive in terms of the amount of enriched uranium produced.
A two-photon photo-excitation process for separating uranium isotopes involves subjecting uranium in the gaseous phase, either in ion form or in a molecular compound, to a source of radiation in order to excite the molecules bearing one of the uranium isotopes but not the molecules bearing the other isotope and then subjecting the excited uranium isotope molecules to further radiation to ionize or dissociate the excited uranium isotope molecules in order to achieve isotope separation. These photo-excitation processes utilize the fact that changes in the nuclear mass of a compound can shift electronic, vibrational, vibronic and rotational energy levels. When the isotopic shift places the absorption spectrum of one isotopic species at a frequency at which another isotopic species is transparent, it is possible to selectively excite the first isotopic species with a laser of sufficiently narrow line width. Photo-excitation processes such as these are disclosed in U.S. Pat. No. 3,433,087 to J. Robieux et al., U.S. Pat. No. 3,772,519 to Levy et al., and British Pat. No. 1,284,620 to Gurs.
The gaseous diffusion and centrifugation processes and all known photo-excitation processes for uranium isotope separation all have in common the fact that the sample or working medium of uranium which is subjected to diffusion or centrifugation or which is irradiated is in the gaseous state or phase.
In prior photo-excitation processes for separating uranium isotopes which have used a working medium in the gaseous state, it has been necessary to operate at extremely low pressures (e.g. less than 5 torr but more typically 1 torr) in order to obtain a sharp resolution of the lines of the uranium ion or compound absorption spectra. If the pressure used is not sufficiently low, the uranium ions or molecules have a tendency to collide with one another and obliterate the distinction between lines of the absorption spectra which are representative of different energy levels. Since the pressure of a gas is directly proportional to its density, the lower the pressure the lower the density of the gaseous molecules and, consequently, the lower the yield of enriched uranium in these photo-excitation processes using a working medium in the gaseous phase. This inability to produce a large yield of the enriched isotope has been one of the major problems with these prior photo-excitation processes.
A second major problem with these prior photo-excitation processes in which a working medium in the gaseous state is irradiated is the back reactions which occur among the gaseous molecules both after the molecules bearing one of the uranium isotopes have been raised to an excited state and after these excited molecules have been ionized or dissociated. The incidence of back reactions is particularly high in the gaseous phase because of the constant motion of the molecules. The collision of any molecule with a uranium isotope-bearing molecule in an excited state may rob that isotopic molecule of its energy thus lessening the number of molecules which can be successfully ionized or dissociated in order to be able to accomplish physical separation of the uranium isotopes. Collisions which occur after the uranium isotope-bearing molecules have become ionized or dissociated cause difficulty in the physical separation or collection of the ionized or dissociated molecules. This back reaction problem of these prior photo-excitation processes also contributes to the failure of these processes to produce practical quantities of enriched uranium.
In summary, the main problem with the previous photo-excitation processes for separating uranium isotopes in which a working medium in the gaseous phase is irradiated is their inability to produce practical amounts of enriched uranium. Contributing factors to this main problem which are directly related to the use of a gaseous phase working medium are the low density of the gaseous molecules at the low pressures at which these processes must be conducted, and the back reactions which occur among the gaseous molecules which lessen the amount of molecules which are ionized or dissociated and which also lessen the amount of molecules which can be physically separated or collected.